From my understanding... the focal length will always remain the same, it just gets cropped off so it appears to be closer, however the depth of field will be affected. Our NEX APS-C sensors have a 1.5 crop factor so you would just simply multiply the focal length and aperture with the crop factor.

The field of view of a 50mm f/1.7 would be similar to 75mm f/2.55, but the depth of field will be more of a 50mm f/2.55 in 35mm format terms.